The present invention relates to a process for the recovery of precious metals from sulfide concentrates or mattes, particularly those containing nickel sulfides. More specifically, the process of the present invention relates to the conversion of precious metals, metal sulfides and nickel sulfides to metal chlorides, and the subsequent recovery of the precious metals and nickel.
Precious metal ores that contain platinum group metals (PGMs) are usually associated with sulfide minerals of copper, nickel, and iron. Typically, these ores have been smelted to separate the sulfide minerals from the gangue. That is, upon smelting, the sulfide minerals and the precious metals collect in a separate molten phase known as matte while gangue is separated as a molten slag. After smelting, the slag is discarded and the matte is cooled, crushed, ground, and leached to remove the base metals. U.S Pat. No. 4,571,262 describes the use of sulfuric acid to leach the matte. This process is known as the Sherritt Gordon process. Canadian Patent No. 970,980, describes the use of hydrochloric acid and chlorine for leaching the matte. This process is known as the Falconbridge chlorine leach process.
The remaining solid residue from the leaching process (often referred to as PGM concentrates), is further treated to recover precious metals. Typically, PGM concentrates are subjected to another leaching step that converts the precious metals to soluble chlorides which then can be refined to pure metals.
Unfortunately, such processes suffer from several major disadvantages. For example, during the leaching of the base metals some of the precious metals are also extracted and an additional step is needed for their recovery from solutions that are rich in base metals. Also, the PGM residue is impure. It contains large quantities of base metals, elemental sulfur and harmful impurities such as arsenic, antimony, selenium, tellurium, bismuth and tin. These impurities end up in the PGM chloride solutions and render it very difficult to refine the PGMs to pure metals. For example, selenium and tellurium impurities are known to react with various organic solvents that are used in the multiple solvent extraction stages in PGM refining operations. This usually causes the formation of an organic "crud" that entraps PGMs, disrupts the solvent extraction and causes losses in PGMs. Bismuth and antimony are difficult to separate from precious metal products and cause loss of purity.
Also, the PGM concentrates (the first leach residue) are not homogeneous materials. Concentrates of PGMs including gold and silver, can be characterized as those which are generally soluble, i.e., platinum, palladium and gold, and those that are generally insoluble, i.e., rhodium, iridium, ruthenium, osmium and silver (rhodium, iridium, ruthenium and osmium are often referred to as "secondary PGMs"). Thus, when PGM concentrates are subjected to leaching, platinum and palladium dissolve while the secondary PGMs remain insoluble. The residue which is rich in secondary PGMs is normally resmelted and dissolved in an additional leaching step. The recovery of secondary PGMs and the special processing that it requires is described in U.S Pat. No. 4,397,689. Unfortunately, there is not a complete split between soluble and insoluble metals. For example, some platinum remains undissolved and is found in the insoluble stream and some ruthenium may dissolve and is found in the soluble stream.
All of the above difficulties render conventional PGM processing tedious, long and complex. Also, the fact that gold dissolves with precious metals is troublesome since it must be completely removed before the separation and refining of the PGMs.
Metals within sulfide ores have also been recovered by chlorination. Generally, metals recovery processes that employ chlorination reactions can be classified in three groups, namely gaseous chlorination, salt chlorination (in the absence of chlorine gas), and chlorination in a molten salt bath in the presence of chlorine gas. In this regard, U.S. Pat. Nos. 4,011,146 and 4,362,607 describe gaseous chlorination processes, U.S. Pat. No. 1,883,234 describes chlorination by salt addition, and U.S. Pat. No. 4,209,501 describes a molten salt extraction processes. U.S. Pat. Nos. 4,353,740, 3,825,651, 3,988,415, and 4,209,501 describe the use of chlorination for precious metals recovery.
As explained in H. Parson's "Low Temperature Dry Chlorination of Sulfide Ores--A Review," CIM Bull. Vol. 71, 196 (March 1978), the reaction between chlorine gas and metal sulfides has been known at least since the early part of the century (see also U.S. Pat. No. 1,388,086 issued Aug. 16, 1921). Many researchers have since tried to create commercial processes, hoping that gaseous chlorination would enable them to treat complex sulfide ores, produce elemental sulfur, and use less energy.
However, there are problems inherent in gaseous chlorination processes. For example, precious metals and especially PGMs are difficult to chlorinate at high yields. Also, when other metal sulfides chlorinate it is difficult to separate between the numerous metal chloride products. In addition, the metal chlorides fuse with the solid residue and cause plugging of the chlorination reactor, and sulfur chlorides form. Furthermore, both the metal chloride and the sulfur vapors are difficult to separate and recover, the reaction produces large volumes of gases which cause large losses of concentrate dust, there is poor recovery yield, and the equipment corrodes. Because of these reasons, precious metals ores that contain other sulfides are often first roasted prior to chlorination. The roasting removes sulfur and converts base metal sulfides to metal oxides that do not chlorinate readily. In this way the problems associated with the separation of sulfur from chloride vapors, the separation of chloride products and the fusion of large quantities of base metal chlorides in the reactor are eliminated. U.S Pat. No. 4,353,740 describes such a procedure for treating gold ores. A similar procedure for the extraction of PGMs is described by the Council for Mineral Technology, private Bag *3015, Randburg, 2125 South Africa in their special publication No. 12 1987 compiled by A. M. Edwards and M. H. Silk. In that publication a chlorination process is described which was developed by Rand Mines (South Africa) and used by Potgiestersrust mines (1930) in which a sulfide rich platinum concentrate was roasted and then chlorinated in the presence of salt to form soluble PGM chlorides. The overall yield of PGMs from this step was 85%. While in 1930 this level of PGM recovery was apparently satisfactory, it is totally unacceptable today.
As is evident from U.S. Pat. No. 589,959 issued Sep. 14, 1887, chlorination with salt in the absence of chlorine gas, dates back to the 19th Century. However, these processes are also often ineffective. For example, the processes use high temperatures, usually ranging from 900.degree. to 1000.degree. C., and have high energy requirements. Furthermore, at these high temperatures the metal chlorides are volatilized and gas scrubber systems are required for their subsequent recovery.
The use of a molten salt bath process has the potential to be advantageous. However, the prior art processes have experienced problems. For example, sulfur monochloride often forms and certain metal chlorides are volatilized from the bath and their vapors mix with sulfur vapors and sulfur monochloride Copper and iron chlorides are typical examples of affected base metal chlorides while all of the precious metal chlorides are highly volatile. U.S. Pat. No. 4,209,501 describes the evaporation of gold from a molten salt bath.
In addition, in the chlorination of nickel containing matte the rates of reaction have been slow and yields have been low. Low yields, of course, are particularly unacceptable when precious metals are involved. To be practical, at least 98%, preferably .gtoreq.99%, of the precious metals should be recovered.
Those mattes containing nickel sulfides have been particularly difficult to chlorinate in chloride melts. For example, U.S. Pat. No. 3,802,870 shows that during chlorination of a nickel sulfide containing matte in a molten salt bath at a temperature ranging from 750.degree. C. to 950.degree. C., the other sulfides in the matte are chlorinated, but not the nickel sulfides. Thus, this type of process must be operated at high temperatures to maintain the nickel containing matte in a molten state.